The thaxtomins are a group of phytotoxins generated by some species of Streptomyces bacteria, such as Streptomyces scabies (the main causal organism of potato common scab). The thaxtomins can cause plant cell necrosis of various plant species.
Five toxins, including thaxtomin A and thaxtomin B, that induce the formation of scabs on potato tubers have been isolated from S. scabies. They are cyclic dipeptides classed as 2,5-Diketopiperazines, with thaxtomin A, the most abundant, having the chemical formula C22H22N4O6. Individual thaxtomins appear to differ only in the presence or absence of N-methyl and hydroxyl groups and their respective substitution sites. For instance, thaxtomin A and thaxtomin B differ only in the presence of a hydrogen at C20 in thaxtomin B, rather than a hydroxyl group.
The pathogenicity of Streptomyces strains is believed to be related to the amount of thaxtomin A they produce, as thaxtomin A is believed to be the most physiologically active. It is synthesized by peptide synthetases encoded by the txtA and txtB genes. The genes involved in thaxtomin biosynthesis are located on a part of the genome called the pathogenicity island, also present in S. acidiscabies and S. turgidiscabies. 
Cellobiose and cellotriose, subunits of cellulose, activate thaxtomin production in some strains. The target of the thaxtomins is unknown, but evidence suggests that they inhibit the growth of plant cell walls, having an apparent ability to inhibit cellulose synthesis in developing plant cells. They are not organ- or plant-specific and cause death in various plant species.
Although studies have indicated that the thaxtomins have many of the biological properties desirable in a commercial herbicide, production on an industrial scale presents problems, such as slow production, low yields, and the need for certain inducers, such as cellobiose.